Age, injury, trauma, and disease can cause degenerative changes in both the joints and bones of the body. At some point, these degenerative changes can become so advanced and/or debilitating that it becomes necessary to replace a damaged joint with a prosthetic device. In such cases, bone cement is often used to secure the prosthetic device to the natural bone. For those suffering from severer osteoporosis, procedures such as vertebroplasty and kyphoplasty use bone cement to stabilize and/or build up the vertebral bodies that have been weakened by compression fractures. These procedures can help prevent further fracturing of the vertebral bodies as well as relieve the pain caused by existing fractures.
When referring to bone cement it should be understood that bone cement includes any type of surgical cement used in any type of surgical procedure including: resorbable cement, bone graft material, bone substitute, bone filler or any other biologically compatible mixture that is highly viscous.
Bone cement is primarily a two component material, the first component being a powder and the other being a liquid. The cement may also include additional ingredients such as stabilizers, one or more antibiotics, contrast agent(s), and/or colorants. Typically, the powdered component is comprised of polymethylmethacrylate (PMMA) which copolymerizes with the liquid component, methylmethacrylate (MMA), upon mixing. The polymerization process can be divided into four different phases: mixing, waiting, working, and setting.
The mixing phase starts the moment the powder and liquid components come into contact with each other. During this phase the cement is thoroughly mixed to reduce the porosity of the cement and increase its mechanical strength. The mixing phase is also characterized by changes in cement viscosity. At the beginning of mixing, the cement viscosity increases slowly. As the polymerization reaction progresses, however, the cement rapidly becomes increasingly viscous.
In the waiting phase, the cement will achieve a suitable viscosity for delivery to the surgical site. At the beginning of the waiting phase, the cement has a sticky dough-like consistency. However, the optimal consistency for delivering the cement in vivo is attained when the cement loses this sticky quality. Loss of stickiness marks the beginning of the working phase.
In the working phase, the cement is no longer sticky and has a viscosity that is high enough to allow penetration into cancellous bone without leaking into the surrounding tissues. The duration of the working phase is relatively short-lived and, in part, depends upon the type of bone cement being used. For example, low viscosity cements have a relatively short working phase while high viscosity cements have a longer working phase. Cements with a longer working phase typically allow a surgeon more time to apply the cement before the cement enters the setting phase and begins to harden. Regardless of the type of cement used, the finite duration of the working phase necessitates an efficient and precise means for its delivery and application.
In the setting phase, the cement hardens completely and attains its full mechanical strength. Hardening is generally a temperature sensitive process and can be influenced by body temperature, the temperature of the operating room, and the temperature of the bone cement material itself. High viscosity cements are sometimes pre-chilled before mixing, which prolongs the working phase as well as the setting phase. Humidity can also affect the working and setting phases of bone cement.
Considering the time dependent relationship between the optimal viscosity for delivery of the cement to the surgical site and the onset of cement hardening, an efficient, effective and convenient means for delivering bone cement is highly desirable.